Two row angular contact bearings have a pair of coaxial races with two pairs of diagonally opposed ball pathways between which two rows of bearing balls run. By diagonally opposed, it is meant that the pathways of each pair are both axially and radially separated, and cover less than 90 degrees, contacting the ball row at opposed quadrants, in effect. This, as opposed to so called full arch ball pathways, in which the pathways cover more than 90 degrees, and are separated only in the axial or radial directions. The thin annular sections along which the balls actually contact the pathways form what is termed a contact angle with one another, and the contact angles of the two pairs of pathways may converge or diverge externally of the bearing. Because of their load supporting and assembly characteristics, such bearings are particularly suitable for vehicle wheel bearings, and an example of such may be found in U.S. Pat. No. 4,179,167 to Lura et al, assigned to the assignee of the current invention.
When assembling the bearing, and particularly when choosing the balls for each ball row, careful account must be taken of manufacturing tolerances. Neither the pathways nor the balls can be ground to exacting, perfect specification in every case, so one must be chosen to match the other. For example, ball pathways that are slightly widely spaced can be filled with balls that are slightly over specification in diameter, or vice versa, and the bearing will operate acceptably. One of the critical parameters that depends on how carefully the balls are matched to the pathways is axial end play, a measure of how far the races will move axially relatively to one another as they run. Closer matching of ball diameter to pathway width reduces end play.
Conventionally, the matching process consists of picking a pair of inner and outer races, referred to as the spindle and hub respectively, and holding them in a gauging jig that aligns them until the two opposed pairs of ball pathways are equally spaced apart. Then, the width of the two pairs of pathways is gauged. The available supply of bearing balls is divided up into a series of discrete classes, one of which has a diameter that is substantially equal to the ideal ball diameter, and the rest of which are larger or smaller than the ideal diameter by an integer multiple of a predetermined increment. In fact the bearing balls in within each class differ in size slightly, due to tolerances, but are arrayed in a normal distribution about the class size. How many classes of balls there are depends on how small the predetermined increment is. That is, an increment half as large will require twice as many ball classes.
Once the ball pathway width is measured, to whatever accuracy the gauge allows, balls are chosen from the class that most closely matches the gauged width. However, since the classes of balls are discrete, not continuous, they do not cover every possible gauged width, and cannot always match that width. The match will be farthest off when the gauged width falls midway between two available ball classes. Conventionally, the ball diameter chosen will be either that from the "too large" class above, or the "too small" class below the midpoint. The first choice has low end play, but the balls are tight between the pathways, which can cause a high preload. The second choice has a lower preload, but higher end play. The obvious way to improve the situation is to halve the increment, and double the number of ball classes. This is more expensive in terms of time and inventory, however.